Ink jet printing systems can generally be divided into two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The ink stream is then perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet, thereby directing the droplet to a gutter for recirculation or to a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled in drop-on-demand systems unless it is to be placed on the recording medium.
Ink jet inks may be water-based, or may employ an alkylene glycol or other solvent base. Regardless of the type of ink used, the surface chemistry of the recording medium largely determines the print quality. Therefore, recording media for ink jet and other printing systems have utilized pigment-containing coatings to sorb the solvent of the ink (i.e., dry the ink) and hold its dye-component to maximize the color development and visual effect of the ink. It is known that highly porous, high surface area pigments are especially effective in this regard, as such pigments maximize the amount of ink dye positioned in the path of light reflected from the substrate to the eye, while at the same time minimizing the dye absorbed in the pigment layer or paper substrate.
The drawback of these highly porous, high surface area pigments has been that they also rapidly build viscosity in aqueous systems and coating compositions as loading levels increase. Consequently, coating suspensions of known highly porous/high surface area pigments can only achieve about 10-20% pigment solids before their viscosities exceed practical levels for processing and application. This disadvantageously limits the amount of high surface area pigment that can be effectively utilized in recording media coatings, thereby lowering the solids content and reducing the weight of the coatings. Coating performance suffers as a consequence.